As a conventional outdoor unit of an air conditioner, there is known an outdoor unit having a structure in which the inside of a casing having a generally rectangular parallelepiped box shape is divided into a fan chamber and a machine chamber by a partition plate extending vertically. This fan chamber mainly includes an outdoor heat exchanger and an outdoor fan disposed therein. In addition, the machine chamber mainly includes a compressor, refrigerant circuit components comprising valves, refrigerant pipes, and the like, and an electric equipment assembly disposed therein. This type of electric equipment assembly includes a control circuit board on which operating control elements are mounted (see Japanese Patent Application Publication No. 09-236284).